Signal transmission in wireless communication systems is subject to fading which often reduces the achievable throughput and data rates or achievable quality-of-service. Transmission environments with obstacles lead to multi-path signal propagation, and the power of combined effective received signal power can diminish reducing the link capacity significantly. In addition, due relative speed between the transmitter and the receiver, or the intermediate objects between the transmitter and the receiver, the fading changes dynamically in time and space.
A typical countermeasure for a fading channel is to employ receiver diversity with multiple receive antennas. Multiple receive antennas are often expensive to implement and subsequently alternative solutions have been sought for. Transmit diversity is an alternative solution in which more than one transmit antenna is used to transmit the signal. Both of these techniques create artificial multi-path channels and the probability that all channels fail simultaneously is significantly reduced, thus improving the quality of the received signal.
One transmit diversity solution is disclosed in U.S. Pat. No. 6,185,258 to Alamouti et al., which is incorporated herein by reference. The Alamouti matrix CAla is shown below in equation (1), with each row corresponding to a transmit antenna, or a beam, and each column corresponding to a symbol period.
                                          C            Ala                    ⁡                      (                                          z                1                            ,                              z                2                                      )                          =                  [                                                                      z                  1                                                                              -                                      z                    2                    *                                                                                                                        z                  2                                                                              z                  1                  *                                                              ]                                    (        1        )            
The Alamouti scheme is called a 2 by 2 space-time block code, as it employs two transmit antennas or beams during two symbol periods. As an alternative of time-division, transmitting different columns during different symbol periods, any other substantially orthogonal division of the available transmission resources may be used, e.g. different frequency subcarriers or Fourier/wavelet waveforms (space-frequency code) or different (spreading) codes (space-code-code) may be used. To stress this multitude of uses of a given code matrix, the term “transmit diversity code” shall be used for codes of the type discussed above, which may be used when a spatial (antenna or beam) dimension is available, together with any substantially orthogonal division of other transmission resources, including time and bandwidth. The transmit diversity of the Alamouti code is two, as taught in the U.S. Pat. No. 6,185,258. The symbol rate is one, since two symbols are transmitted in two time slots. The code formed according to equation (1) is orthogonal, in the sense that, when multiplied together with its Hermitian transpose, it yields a scaled identity matrix. The Hermitian transpose of a matrix A, denoted by AH, is the complex conjugate transpose of A. The transpose of a matrix is derived by reversing the row and column indices of the matrix. The identity matrix, denoted “I”, is a matrix with each element on its diagonal equal to unity and all other elements each to zero. Accordingly, for an orthogonal-based matrix A, it holds that AHA=AAH=k I, for some real value k. The orthogonality of the Alamouti matrix enables separate decoding of the two symbols, in such a way that that symbols do not interfere with each other.
The Alamouti transmit diversity is optimized for channels in which there is little or no intersymbol interference (ISI) on the channel. ISI distorts the received signal and exacerbates reception, thus reducing signal quality. The time delayed signals, also known as temporal multi-path components, can be also advantageous. In CDMA systems one may, as an example, employ a separate transmit diversity block code decoder for each multi-path component, and then combine the output using any suitable diversity combining method, including as an example equal gain combining, or maximal ratio combining. Alternatively, an equalizer may be used to combine the multi-path propagated signals, and possibly to simultaneously remove inter-symbol-interference. Lindskog and Paulraj have proposed in “A Transmit Diversity Scheme for Channels with Intersymbol Interference”, Proc. IEEE ICC2000, 2000, vol. 1, pp. 307-311, an orthogonal transmit diversity block code that, unlike the Alamouti code, is effective on ISI channels. This paper is incorporated herein by reference.
Orthogonal transmit diversity codes suffer from rate limitation problems, as taught in O. Tirkkonen and A. Hottinen, “Complex space-time block codes for four Tx antennas” in Proc. Globecom 2000, San Francisco, USA, November/December 2000, incorporated herein by reference. As an example, the maximal symbol rate for an orthogonal transmit diversity code with four transmit antennas or beams is ¾. When the rate loss is not desired the code orthogonality has to be sacrificed. Indeed, O. Tirkkonen, A. Boariu, A. Hottinen, “Minimal non-orthogonality space-time code for 3+transmit antennas,” in Proc. IEEE ISSSTA 2000, September, NJ, USA, teach one such method (e.g. the ABBA code). In this code the signal is transmitted in using the transmit diversity code matrix
                              C          NOSTBC                =                  [                                                                      z                  1                                                                              -                                      z                    2                    *                                                                                                z                  3                                                                              -                                      z                    4                    *                                                                                                                        z                  2                                                                              z                  1                  *                                                                              z                  4                                                                              z                  3                  *                                                                                                      z                  3                                                                              -                                      z                    4                    *                                                                                                z                  1                                                                              -                                      z                    2                    *                                                                                                                        z                  4                                                                              z                  3                  *                                                                              z                  2                                                                              z                  1                  *                                                              ]                                    (        2        )            
It is seen that the code comprises as sub-matrices the Alamouti code. The aforementioned paper is incorporated herein by reference. The code described above yields good performance in a fading channel but due to the structure of the non-orthogonality, there is an inherent performance loss in correlated channels or in Ricean channels, where known orthogonal transmit diversity codes perform better. The performance of non-orthogonal codes, exemplified by (2), can be improved by employing possibly matrix valued constellation rotations, as discussed in O. Tirkkonen, “Optimizing space-time block codes by constellation rotations,” Finnish Wireless Communications Workshop, October 2001, which is incorporated here by reference. The idea is that if the symbols in different orthogonally encoded blocks, exemplified by the pairs z1,z2 and z3,z4 in (2) are taken from different constellations, the performance of the code is much improved. This can be realized by constellation rotations.
A simpler, limited diversity space-time code construction has been proposed for WCDMA systems. The orthogonal code is called STTD-OTD in 3GPP document TSGR1#20(01)-0578, incorporated herein by reference. It combines two Alamouti codes in such a way that the symbol rate is one (with four transmit antennas), but so that the system only enjoys limited diversity order. The transmission code matrix is
      C          STTD      ⁢              -            ⁢      OTD        =      [                                        z            1                                                z            1                                                z            2                                                z            2                                                            -                          z              2              *                                                            -                          z              2              *                                                            z            1            *                                                z            1            *                                                            z            3                                                -                          z              3                                                            z            4                                                -                          z              4                                                                        -                          z              4              *                                                            z            4            *                                                z            3            *                                                -                          z              3              *                                            ]  
With four antennas the diversity order is only two when four is the maximum achievable. It is noted that the STTD-OTD code above contains two Alamouti blocks, and it can be written using the Alamouti matrix, given earlier, after changing the column indices 2 and 3. Alternatively, to obtain essentially the same diversity as with STTD-OTD one may combine antenna hopping and the Alamouti code, in which case the space-time matrix is
                              C                      STTD            ⁢                          -                        ⁢            AHOP                          =                  [                                                                      z                  1                                                                              -                                      z                    2                    *                                                                                                                                                                                                                                                                                    z                  2                                                                              z                  1                  *                                                                                                                                                                                                                                                                                                                                                                                                                              z                  3                                                                              -                                      z                    4                    *                                                                                                                                                                                                                                                                                    z                  4                                                                              z                  3                  *                                                              ]                                    (        3        )            
It is seen that the matrix contains four symbols and occupies four time slots, and hence the symbol rate is one, although all symbols are not transmitted from all antennas, thus limiting the achievable diversity to two.
Transmit diversity block codes have been designed also for parallel high rate transmission over fading channels, as taught by O. Tirkkonen and A. Hottinen, “Improved MIMO transmission using non-orthogonal space-time block codes,” in Proc. Globecom 2001, November/December 2001, San Antonio, Tex., USA, incorporated herein by reference. In this method, two transmit antennas and two receive antennas are used advantageously to obtain both transmit/receive diversity benefit and increased data or symbol rate.
High rate space-time transmission concepts have been considered also for future WCDMA systems. Indeed, in the Third Generation Partnership Program (3GPP) document “Improved Double-STTD schemes using asymmetric modulation and antenna shuffling” TSG-RAN Working Group 1 (TSGR1#20(01)-0459) by Texas Instruments (incorporated herein by reference), proposed parallel transmission of Alamouti codes using four transmit antennas and two or four receive antennas. Although this method improves the symbol rate by a factor of two it obtains only limited diversity order, which limits the performance and realizable data rates.